1. Field of the Invention
The present invention is directed to the field of surface cleaning and etching of silicon substrates and more specifically to an improved apparatus for enhancing those processes.
2. Description of the Prior Art
The use of ultrasonic energy to generate cavitation in cleaning solutions and thereby enhance cleaning action is a common, well-established practice and is described in U.S. Pat. Nos. 3,198,489; 3,240,963; and 4,401,131.
Ultrasonic agitation has also been used to enhance the ability of etching solutions to etch materials under certain conditions. One description of such use is included in a paper entitled TEM Observation of Pyramidal Hillocks Formed On (001) Silicon Wafers During Chemical Etching, by Fumio Shimura, J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY, April, 1980, pgs. 910-913.
Both cleaning and etching processes are important in the production of many types of semiconductor devices. However, in the past, the quality achieved by the application of ultrasonic energy has been limited by the types of sources used in high-energy ultrasonic equipment that is commercially available and due to the fact that the prior art equipment operated mostly in the 20-50 KHz frequency range.
The basic mechanisms associated with ultrasonic cavitation are understood to be due to microscopic cavities or voids that exist within liquids. Upon application of a high amplitude ultrasonic pressure wave, a cavity will grow by extracting energy from the sonic field and concentrating it in the vicinity of the void. The cavity grows to a size where the motion of the cavity wall resonates with the driving force of the incident wave motion. After some time, the motion of the cavity wall becomes unstable and the cavity collapses. The energy stored in the region around the wall causes a transient, localized turbulent flow accompanied by high stresses. It is this combination of turbulence and high stresses that produces the beneficial action useful in cleaning or etching.
Theoretical studies have indicated that the relationship between cavity radius and linear resonant frequency, in water, is as shown in FIG. 1. This relationship indicates that at a frequency of 1 MHz, for instance, the radius of the resonant cavity should be about 4 microns, as indicated by the dashed lines. The dependency of resonant cavity size to frequency is basic to the benefits expected from ultrasound to process semiconductor devices. By achieving a smaller cavity size, there is an improved ability to clean or etch structures with low micron sized definition. Additionally, since the smaller cavity size inherently stores less energy, less energy is released on collapse of the void and the result is a milder cleaning action than would occur by cavitation produced by KHz frequencies.
A conventional (prior art) ultrasonic cleaning apparatus is shown in FIG. 2 to illustrate some of the limitations present in the art. A liquid cleaning solution 12 is contained in a stainless steel tank 10. Piezoelectric transducers 14 are bonded to the bottom of the tank and may number one or more. Those transducers 14 are usually three or four inches in diameter and approximately 1/4 to 1/2 inch thick. It is very common that the transducer 14 will resonate somewhere in the range of 25 to 50 KHz. The transducer 14 is driven by an electrical power oscillator 16 that may be operated directly from a 110 volt AC (60 Hz) line. The resulting waveform applied to the transducer 14 is a pulse of sinusoidal oscillations (25 KHz to 50 KHz) modulated at a 60 Hz rate. This type of construction minimizes the cost of a power supply and at the same time, by modulating the wave motion radiated into the tank, prevents the build up of any steady-state, standing wave patterns that would otherwise result in dead spots.
The major disadvantage of the conventional tank is that it cannot be operated at MHz frequencies to obtain the desired low micron size cavitation. For instance, even with thin transducers, the stainless steel tank 10 becomes extremely lossy at high frequencies. In addition, if cleaning or etching is to be performed with solutions that attack the stainless steel tank 10, the corrosive liquid has to be contained in a beaker which is immersed in a water bath in the tank. A significant loss of energy takes place as a result of reflections from the boundry surfaces defined by the beaker.
In U.S. Pat. No. 3,893,869, an attempt was made to avoid the use of transducers radiating through the tank wall by simply immersing high-frequency transducers directly into a cleaning bath. Such an arrangement would not be suitable for an etching process since the liquid would most likely attack and destroy the transducer material or the transducer electrodes.